Method and system for real time correction of an image

ABSTRACT

The present invention relates to a system and method for real time correction of light output and/or colour of an image displayed on a display device. The system comprises:  
     a display device ( 1 ) comprising an active display area ( 6 ) for displaying the image, an image forming device ( 2 ) and an electronic driving system ( 4 ) for driving the image forming device ( 2 ),  
     an optical sensor unit ( 10 ) comprising an optical aperture ( 21 ) and a light sensor ( 22 ) having an optical axis, to make optical measurements on a light output from a representative part of the active display area ( 6 ) of the image forming device ( 2 ) and generating optical measurement signals ( 11 ) therefrom,  
     a feedback system ( 12 ) receiving the optical measurement signals ( 11 ) and on the basis thereof controlling the electronic driving system ( 4 ).  
     The optical aperture ( 21 ) of the optical sensor unit ( 10 ) has an acceptance angle such that at least 50%, alternatively 60%, alternatively 70%, alternatively 75% of the light received by the sensor ( 22 ) comes from light travelling within 15° of the optical axis of the light sensor ( 22 ).

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to a system and method for realtime correction of light output and/or colour of an image displayed on adisplay device of the type having a display area which has a brightnessversus voltage characteristic which is dependent on temperature andageing of the display device, such as for example LCD (liquid crystaldisplay) devices, plasma devices, CRT, electronic displays, rearprojection systems.

[0002] It applies more particularly, but not exclusively, totransmissive type liquid crystal displays intended to be observed alonga given axis of vision. Such is the case in particular in computerscreens and in instrument panels of vehicles such as aircraft.

[0003] More particularly, when a plurality of, for example, LCD screensare put together to form one or a plurality of images, it is desiredthat each of the LCD screens has the same appearance.

BACKGROUND OF THE INVENTION

[0004] At present, LCD displays are often equipped with means foradjusting the contrast, whereby such adjustment is carried out at thebeginning of a phase of use of the display. The contrast is not adjustedanymore during the use of the display.

[0005] JP-8292129 describes a separate measurement device which is puton the display at the moments when measurements are carried out only.Such a sensor for measuring display light has any one of an opticalsystem, an aperture and an optical fibre which are mounted on a displaysurface. Among all the light beams emitted from the display surface,only the light corresponding to the optimum observing angle is appliedinto a sensor element. The sensor described does not allow forcompactness and integration. The height of the sensor is large, as, inorder to apply only the light corresponding to the optimum observingangle to the sensor element, the sensor element has to be placed quitesome distance away from the aperture, and thus from the screen. Thesensor also covers a quite large area. The sensor described is notdesigned for real-time use in parallel with the application displayed onthe screen. For instance it would be impractical for a mobile phonedisplay.

[0006] For some applications it is preferred to have a measurementdevice which can stay on the display all the time, so as to be able tocontinuously measure the light output, and thus continuously correct thelight output and/or the colour of a displayed image.

[0007] From EP-0313331 an apparatus for controlling the brightness andcontrast of a liquid crystal display during its actual use is known. Apredetermined light level is produced and transmitted through an LCDpanel by a light emitting diode. At the other side of the panel, thelight transmitted through the panel is sensed by a sensor, andcorrespondingly a signal representative thereof is produced. This signalis monitored in real time and is used for adjusting in real time thegrey scale levels of the panel. The light emitting diode and the sensorare positioned on opposite surfaces of the LCD panel, under a lightshielding material. This light shielding material shields the viewerfrom the light generated in that portion of the screen, but also shieldsthe sensor from ambient light. A disadvantage of this solution is thatit cannot be retrofitted on existing screens. Furthermore, such acovered test pixel, although not having problems with ambient light,masks too much of the LCD.

[0008] U.S. Pat. No. 5,162,785 and DE-4129846 also describe a device foroptimising the contrast of an LCD. The LCD panel comprises a displayarea and a separate test zone. A sensor observes the test zone. Both thesensor and the test zone are protected from ambient light by an opticalmask.

[0009] U.S. Pat. No. 5,490,005 describes a light sensor placed on an ownlight source (LED) of a display device. A first disadvantage is thatthis cannot be implemented on commercially available LCD panels. Afurther disadvantage is that the feedback loop does not incorporate thebacklight that is used for the active image content, and therefore thisstructure is not very precise: it does not measure the overall result ofall image defining elements (such as the LCD itself, the backlight,filters, temperature etc.).

SUMMARY OF THE INVENTION

[0010] It is an object of the present invention to overcome thedisadvantages mentioned above.

[0011] This object is accomplished by a method and a system according tothe present invention.

[0012] The present invention provides a system for real time correction,through optical feedback, of light output and/or colour of an imagedisplayed on a display device. Light output includes backlight, contrastand/or brightness. The system according to the present inventioncomprises:

[0013] a display device comprising an active display area for displayingthe image, an image forming device, such as e.g. a transmissive orreflective LCD in the case of an LCD device or a phosphor in case of aCRT, and an electronic driving system for driving the image formingdevice,

[0014] an optical sensor unit comprising an optical aperture and a lightsensor having an optical axis, to make optical measurements on a lightoutput from a representative part of the active display area of theimage forming device and generating optical measurement signalstherefrom,

[0015] a feedback system receiving the optical measurement signals andon the basis thereof controlling the electronic driving system. Theoptical aperture of the optical sensor unit has an acceptance angle suchthat at least 50% of the light received by the sensor comes from lighttravelling within 15° of the optical axis of the light sensor (that isthe acceptance angle of the sensor is 30°). In other words theacceptance angle of the sensor is such that the ratio between the amountof light used for control which is emitted or reflected from the displayarea at a subtended acceptance angle of 30° or less to the amount oflight used for control which is emitted or reflected from the displayarea at a subtended acceptance angle of greater than 30° is X:1 where Xis 1 or greater. Under some circumstances it may be advantageous to havean acceptance angle such that at least 60%, alternatively at least 70%or at least 75% of the light received by the light sensor comes fromlight travelling within 15° of the optical axis of the light sensor.

[0016] In another aspect of the invention a system for real timecorrection of light output and/or colour of an image displayed on adisplay device is provided, comprising:

[0017] a display device comprising an active display area for displayingthe image, an image forming device, such as e.g. a transmissive orreflective LCD in case of an LCD device or a phosphor in case of a CRT,and an electronic driving system for driving the image forming device,

[0018] an optical sensor unit comprising an optical aperture and a lightsensor having an optical axis, to make optical measurements on a lightoutput from a representative part of he active display area of the imageforming device and generating optical measurement signals therefrom, and

[0019] a feedback system receiving the optical measurement signals andon the basis thereof controlling the electronic driving system, theoptical aperture of the optical sensor unit has an acceptance angle suchthat light received at the sensor at an angle with the optical axis ofthe light sensor equal to or greater than 10° is attenuated by at least25%, light received at an angle equal to or greater than 20° isattenuated by at least 50 or 55% and light arriving at an angle equal toor greater than 35° is attenuated by at least 80 or 85%.

[0020] The system according to the present invention is meant to be usedin real time, thus during display of a main application. No test patternis necessary, although a test pattern may be used. The main applicationis not disturbed when the measurement in made.

[0021] The optical measurements are non-differential, i.e. ambient lightand real light emitted by the active display area are not measuredseparately. Direct ambient light is not measured, nor does it influencethe measurement appreciably. Indirect ambient light (i.e. ambient lightreflected by the display) has a contribution in the total luminanceoutput of the electronic display, and will be measured.

[0022] In case it is the intention to adjust the luminance of a displayrelative to the ambient light, the combination of the invention with aseparate ambient light sensor is possible. In that case, a systemaccording to the present invention measures the luminance emitted by thescreen, and the ambient light sensor measures the ambient light. Thedisplay's luminance can then be adjusted in proportion to the differencebetween both.

[0023] Preferably, the optical measurements are luminance measurements.The light output correction may then comprise luminance and/or contrastcorrection. The optical measurements may also be colour measurements, inwhich case a colour correction may be carried out.

[0024] The feedback system preferably comprises a comparator/amplifierfor comparing the optical measurement signals, measured luminance orcolour values, with a reference value, and a regulator for regulating abacklight control and/or a video contrast control and/or a videobrightness control and/or a colour temperature, so as to reduce thedifference between the reference value and the measured value and bringthis difference as close as possible to zero.

[0025] The optical sensor unit of the present invention preferablycomprises a light guide between the optical aperture and the lightsensor. This light guide may be e.g. a light pipe or an optical fibre.

[0026] Preferably, the representative part of the active display area ofthe image forming device is less than 1% of the total area of the activedisplay area of the image forming device, preferably less than 0.1%, andstill more preferred less than 0.01%.

[0027] According to a preferred embodiment, the optical aperture of theoptical sensor unit masks a portion of the active display area, whilethe light sensor itself does not mask any part of the active displayarea. The light output from the front face of the active display area ofa display device is continuously measured with a minimal coverage of theviewed image. The light sensor may be brought to the back of the displayarea or to a side thereof, thereby needing a height above the screenarea preferably less than 5 mm. Therefore, a distance between theoptical aperture and the light sensor, needed to reject ambient lightduring measurement, is not created by a distance out of the screen.

[0028] The area measured on the screen is composed of a number of activepixels of the active display area. The area of active pixels measured onthe screen is preferably not larger than 6 mm×4 mm. For example for amobile phone screen, with typical dimensions of the active display areaof 50 mm×80 mm (third generation mobile phone), a measurement zone of 6mm×4 mm constitutes 0.6% of that active display area. For a laptopscreen with an active display area with dimensions of 2459 mm×1844 mm (a12.1 inch screen), a measurement zone of 6 mm×4 mm constitutes 0.0005%of that active display area.

[0029] No dedicated test pixels are necessary, any pixels in the activedisplay area can be used for carrying out optical measurementsthereupon. A test patch may be generated and superimposed on the activepixels viewed by the sensor, or the sensor may view a part of the realactive image. This makes it possible for the system to be retrofitted onany existing display devices. It can be combined with standard displayssuch as AM-LCD (Active Matrix Liquid Crystal Display) without the needfor a dedicated light source or backlight. Furthermore, parts of thedisplay device, such as the screen, can be easily replaced.

[0030] Preferably, a housing of the optical sensor unit stands out abovethe active display area by a distance lower than 0.5 cm.

[0031] The present invention also provides a method for real timecorrection, through optical feedback, of light output and/or colour ofan image displayed on a display device. The method comprises the stepsof:

[0032] displaying the image on an active display area on the displaydevice,

[0033] making optical measurements on light emitted from arepresentative part of the active display area and generating opticalmeasurement signals therefrom, and

[0034] controlling the display of the image on the active display areain accordance with the optical measurement signals. The step of makingoptical measurements comprises selecting light such that the ratiobetween the amount of light used for control which is emitted orreflected from the display area at a subtended acceptance angle of 30°or less to the amount of light used for control which is emitted orreflected from the display area at a subtended acceptance angle ofgreater than 30° is X:1 where X is 1 or greater.

[0035] In a method according to the present invention, comprising thesteps of

[0036] displaying the image on an active display area on the displaydevice,

[0037] making optical measurements on light emitted from arepresentative part of the active display area and generating opticalmeasurement signals therefrom, and

[0038] controlling the display of the image on the active display areain accordance with the optical measurement signals,

[0039] the step of making optical measurements comprising selectinglight for use in the control step by attenuating light travellingat-angles with a normal to the active display area which are equal to orlarger than 10° by at least 25%, attenuating light travelling at angleswith a normal to the active display area which are equal to or largerthan 20° by at least 50 or 55%, and attenuating light travelling atangles with a normal to the active display area which are equal to orlarger than 35° by at least 80 or 85%.

[0040] By the method of the present invention, a controlled light outputand/or correct absolute luminance and colour (Y, x, y) readings can beobtained from a display device.

[0041] The method for correction of an image is used in real time, i.e.in parallel with a running application. The method is intervention free,it does not require input from a user.

[0042] Preferably, the optical measurements carried out are luminancemeasurements. In that case, light output correction may compriseluminance and/or contrast correction. Alternatively, the opticalmeasurements carried out are colour measurements, in which case lightoutput correction comprises colour correction of the displayed image.

[0043] Controlling the display of the image in accordance with theoptical measurement signals is preferably done by comparing themeasurement signals with a reference value, and regulating a backlightcontroller and/or video contrast control and/or video brightness controland/or colour temperature so as to reduce the difference between thereference value and the measurement signals and bring this difference asclose as possible to zero.

[0044] Preferably, the step of making optical measurements furthermorecomprises a step of transmitting the light emitted from the activedisplay area from within the active display area to outside the activedisplay area.

[0045] Other features and advantages of the present invention willbecome apparent from the following detailed description, taken inconjunction with the accompanying drawings, which illustrate, by way ofexample, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0046]FIG. 1A is a top view and FIG. 1B is a front view of a part of anLCD screen provided with an optical sensor unit according to the presentinvention.

[0047]FIG. 2 shows a first embodiment of an optical sensor unitaccording to the present invention, the unit comprising a light guidebeing assembled of different pieces of PMMA.

[0048]FIG. 3 shows a second embodiment of an optical sensor unitaccording to the present invention, the unit comprising a light guidewith optical fibres.

[0049]FIG. 4 shows a third embodiment of an optical sensor unitaccording to the present invention, the unit comprising a light guidemade of one single piece of PMMA.

[0050]FIG. 5 shows the light guide of FIG. 4, this light guide beingcoated with a reflective coating.

[0051]FIG. 6 shows the light guide of FIG. 4, this light guide beingpartially coated with a reflective coating, and the light guide beingshielded from ambient light by a housing.

[0052]FIG. 7 shows a measurement set-up to measure angle dependency ofoptical attenuation in a light guide.

[0053]FIG. 8 illustrates how to measure the vertical angle dependency ofthe optical attenuation in a light guide.

[0054]FIG. 9 illustrates how to measure the horizontal angle dependencyof the optical attenuation in a light guide.

[0055]FIG. 10A and FIG. 10B show the results for respectively thevertical and the horizontal angle dependency of the optical attenuationin a light guide, measured as illustrated in FIG. 7-FIG. 9.

[0056] In the different drawings, the same reference figures refer tothe same or analogous elements.

DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

[0057] The present invention will be described with respect toparticular embodiments and with reference to certain drawings but theinvention is not limited thereto but only by the claims. The drawingsdescribed are only schematic and are non-limiting. In the following theacceptance angle of a sensor refers to the angle subtended by theextreme light rays which can enter the sensor. The angle between theoptical axis and the extreme rays is therefore usually half of theacceptance angle.

[0058]FIG. 1A and FIG. 1B are a top view and a front view respectivelyof a part of an LCD display device 1 provided with an embodiment of anoptical sensor unit 10 according to the present invention.

[0059] An LCD display device 1 comprises an LCD panel 2 and anelectronic driving system 4 for driving the LCD panel 2 to generate anddisplay an image. The display device 1 has an active display area 6 onwhich the image is displayed. The LCD panel 2 is kept fixed in an LCDpanel bezel 8.

[0060] According to the present invention, a display device 1 isprovided with an optical sensor unit 10 to make optical measurements ona light output from a representative part of the LCD panel 2. Opticalmeasurements signals 11 are generated from those optical measurements.

[0061] A feedback system 12 receives the optical measurement signals 11,and controls the electronic driving system 4 on the basis of thosesignals.

[0062] Several ways exist to realise the optical sensor unit 10. In allcases, the optical sensor unit 10 is permanently or removably fixed to(or adjacent to) the active display area 6. The whole of the opticalsensor unit 10 can be calibrated together and can also beinterchangeable.

[0063] Typically, the optical sensor unit 10 has a light entrance planeor optical aperture 21 and a light exit plane 23. It can also haveinternal reflection planes. The light entrance plane 21 preferably has astationary contact with the active display area 6 which is light tightfor ambient light. If the contact is not light tight it may be necessaryto compensate for ambient light by using an additional ambient lightsensor which is used to compensate for the level of ambient light.

[0064] Preferably, the optical sensor unit 10 stands out above theactive display area a distance D of 5 mm or less.

[0065] According to a first embodiment, as shown in FIG. 2, the opticalsensor unit 10 comprises an optical aperture 21, a photodiode sensor 22and in between, as a light guide 34, made from, for example, massivePMMA (polymethyl methacrylate) structures 14, 16, 18, 20, of which onepresents an aperture 21 to collect light and one presents a light exitplane 23. PMMA is a transparent (more than 90% transmission), hard andstiff material. The skilled person will appreciate that other materialsmay be used, e.g. glass.

[0066] The massive PMMA structures 14, 16, 18, 20 serve for guidinglight rays using total internal reflection. The PMMA structures 14 and18 deflect a light bundle over 90°. The approximate path of two lightrays 24, 26 is shown in FIG. 2.

[0067] The oblique parts of PMMA structures 14 and 18 are preferablymetallised 28, 30 in order to serve as a mirror. The other surfaces donot need to be metallised as light is travelling through the PMMAstructure using total internal reflection.

[0068] In between the different PMMA structures 14, 16, 18 and 20 thereis an air gap. At these interfaces, stray light (which is light notemitted by the display device) can enter the light guide 34.

[0069] This first embodiment has the disadvantage that it is rathercomplex to realise. Furthermore, stray light or ambient light can enterthe light guide 34 and thus diminish the signal to noise ratio of themeasured light output.

[0070] A second embodiment of an optical sensor unit 10 according to thepresent invention is shown in FIG. 3. It is a fiber-opticimplementation. The optical sensor unit 10 comprises an optical aperture21 and a light sensor 22, with a bundle 32 of optical fibrestherebetween. The optical fibres are preferably fixed together orbundled (e.g. glued), and the end surface is polished to accept lightrays under a limited angle only (as defined in the attached claims).

[0071] This embodiment shows the disadvantage that bending the bundle ofoptical fibres with a small radius is often not practical. Therefore,the distance D the optical sensor unit 10 extends out of the displayarea 6 of the display device 1 is quite large. Furthermore, lightleakage can be present, especially at the surface where the optic fibreis bent over 90°. Ambient light may enter the light guide 34, althoughthis can easily be limited by applying a light screening materialdirectly or indirectly to the surface of the fibre bundle. By “directly”is meant that the screening material and the fibre bundle arenon-separable, e.g. damped metal on the fibre bundle. By “indirectly” ismeant that the screening material and the fibre bundle are separable,e.g. a removable flexible pipe with inside damped metal, which is placedaround the fibre bundle.

[0072] A third and preferred embodiment of an optical sensor unitaccording to the present invention is shown in FIG. 4-FIG. 6. In thisembodiment, the optical sensor unit 10 comprises a light guide 34 madeof one piece of PMMA. The optical sensor unit 10 furthermore comprisesan aperture 21 at one extremity of the light guide 34, and a photodiodesensor 22 or equivalent device at the other extremity of the light guide34. The light guide 34 can have a non-uniform cross-section in order toconcentrate light to the light exit plane 23.

[0073] Light rays travel by total internal reflection through the lightguide 34. At 90° angles, the light rays are deflected by reflectiveareas 28, 30, which are for example metallised to serve as a mirror, asin the first embodiment. The structure of this light guide 34 is rigidand simple to make.

[0074] In an improvement of the structure (see FIG. 5), a reflectivecoating 36 is applied directly or indirectly (i.e. non separable orseparable) to the outer surface of the light guide 34, with exception ofthe areas where light is coupled in (aperture 21) or out (light exitplane 23). The reflection coefficient of this reflective coatingmaterial 36 is 0.9 or lower. The coating lays at the surface of thelight guide 34 and may not penetrate in it.

[0075] In this case, ambient light is very well rejected. At the sametime, the structure provides a narrow acceptance angle: light rays thatenter the light guide 34 under a wide angle to the normal to the activedisplay area 6, such as the ray represented by the dashed line 38, willbe reflected and attenuated much more (because the reflectioncoefficient being 0.9 or lower) than the ray as represented by thedotted line 40 which enters the structure under a narrow angle to thenormal to the active display area 6.

[0076] The structure can further be modified to change the acceptanceangle, as shown in FIG. 6. By selectively omitting the reflective layer36 on the surface of the light guide 34, at places where the structureis not exposed to ambient light (e.g. where it is covered by a displayhousing 42), the light rays travelling under a large angle to the axisof the light guide 34 (or to the normal to the active display area 6)can be made to exit the optical sensor unit 10, while ambient lightcannot enter the light guide 34.

[0077] In this way, light rays that enter the light guide 34 under awide angle to the normal to the active display area 6, such as a lightray represented by dashed line 38, will be further attenuated and evenbe allowed to exit the light guide 34. Light rays that enter the lightguide 34 under a small angle to the normal of the active display area 6,such as a light ray represented by dotted line 40, will be lessattenuated and will only leave the light guide 34 at the level of thelight exit plane 23 and photodiode sensor 22. Therefore, the light guide34 is much more selective as a function of entrance angle of the lightrays. This means that this light guide 34 realises a narrow acceptanceangle.

[0078] By the small acceptance angle of the optical sensor unit 10according to the present invention, it is avoided that ambient lightenters the photodiode sensor 22, and this without having to shield fromthe ambient light neighbouring pixels to the pixels on which themeasurement is done. Also light emitted by the LCD screen at shallowangles to its surface do not enter the sensor. Light emitted from LCDdisplays at angle away from the normal to the surface are oftendistorted in luminance and colour.

[0079] In order to measure the acceptance angle characteristics of anoptical sensor unit 10, test conditions and equipment as hereinafterdescribed, have been used. A light source 44 with white uniformity lightoutput, such as a Hoffman light source type LS-65-GF/PS, has been placedin a dark room 46. A light guide 34 according to the present inventionis placed with its aperture 21 in front of the light guide 34, in firstinstance such that light is entering the aperture 21 in a directionperpendicular to the aperture 21. A preferred requirement of themeasurement set-up is that, the distance d between the light source 44and the aperture 21 of the light guide 34 is equal to or larger than 50times the width c of the aperture surface of the light guide 34. This isto limit the angular spread of light entering the light guide. Forexample, the width c of the aperture 21 may be 3 mm, and the distancebetween the light source 44 and the aperture 21 of the light guide 34may be 17 cm. In this way, most incoming light rays for practicalpurposes are perpendicular to the entrance of the optical sensor unit10. Light coming out of the light guide 34 at the light exit plane 23 iscaptured by a photodiode 22, and the measured light is converted to avoltage by a light-to-voltage converting circuit 47 (linear circuit),which voltage is measured by a digital voltmeter 48, such as e.g. a typePM2525 of Philips. A flat table is provided, of which a portion canrotate in the plane of the table around a vertical axis. A protractor(not represented), e.g. supplied by Helios, is provided to measure therotation of the rotatable part of the table.

[0080] The light guide 34 is fixed to the rotatable part of table toallow rotation of the light guide 34 in a plane parallel to the lineconnecting the light source 44 and the aperture 21 of the light guide34. After the alignment, the light source 44 is fixed to thenon-rotatable part of the table. Alternatively, the light guide 34 maybe fixed to the fixed part of the table, and the light source 44 fixedto the rotatable part of the table.

[0081] Measurements at 0° means that light entering the light guide 34is travelling parallel to the optical axis of the light source 44. Inthis position, the light input of the optical system is at maximum.Measurements at 90° means that the light entering the light guide 34 isperpendicular to the optical axis of the light source 44. In thisposition, the light input of the optical system is at minimum. Note thatan measured at an angle a° refer to an acceptance angle of 2a° becausethe acceptance angle is the subtended angle of extreme rays.

[0082] A suitable method of measurement is as follows. The rotating partof the rotating table is rotated with 1° (around point e, see FIG. 8)until 15° is reached. A protractor is used therefor. Then steps of 5°are applied until 50° is reached. Finally steps of 10° are applied up to90°. At every step, the corresponding measured voltage is noted.

[0083] Since the angle is two-dimensional, the position of the entranceof the optical system is preferably rotated in a perpendicular directiontoo (see FIG. 9). The amount of steps taken depends on the symmetricalproperties of the optical sensor unit 10.

[0084] Since the light guide 34 usually has symmetrical properties, thegraphs obtained (FIG. 10A and FIG. 10B) can be mirrored around 0°. Everymeasurement has to be done in a dark room, so that there are no externalinfluences of light. The ambient temperature is typically chosen to bethat relevant for the operation of the LDC, e.g. 25°.

[0085]FIG. 10A and FIG. 10B show the measurement results, relevant tothe characteristics determined with horizontal and the vertical rotationrespectively. With linear interpolation, the measured values in volt aretransformed to values in percent. The graphs show narrowcharacteristics.

[0086] When looking at the horizontal characteristic, when V_(%) dropsto 50% of the light received at 0°, the angle is about 15°. For lightrays arriving at 35°, only about 10% of the light is received.

[0087] The graph for the vertical characteristic shows no realdifferences with the graph for the horizontal characteristics.

[0088] As shown in the graphs the acceptance angle of a light guideaccording to the present invention is such that light arriving at anangle of 10° or greater to the optical axis (20° acceptance angle) isattenuated by at least 25%, light arriving at an angle of 20° or greater(acceptance angle 40°) is attenuated by at least 50 or 55% and lightarriving at an angle of 35° or greater (acceptance angle of 70°) isattenuated by at least 80 or 85°.

[0089] Accordingly it has been found that the step of making opticalmeasurements can comprise selecting light for use in the control step byattenuating light travelling at angles with a normal to the activedisplay area which are equal to or larger than 10° by at least 25%,attenuating light travelling at angles with a normal to the activedisplay area which are equal to or larger than 20° by at least 50 or55%, and attenuating light travelling at angles with a normal to theactive display area which are equal to or larger than 35° by at least 80or 85%.

[0090] The area under the curves between two angles gives a value forthe amount of light accepted between these angles. One aspect of thepresent invention is to reduce the effect of light which comes fromangles well away from the normal to the display area (as this may bedistorted in hue and/or luminance). Hence, in accordance with anotheraspect of the present invention the ratio of the amount of lightreceived travelling at angle of 15° or less to the normal to the displayarea to the amount of received light travelling at an angle of greaterthan 15° to the normal is X:1 where X is 1 or greater. The physicalarrangement of the optical aperture and the length of the light guideprovide this exclusion of light at high excident angles.

[0091] While the invention has been shown and described with referenceto preferred embodiments, it will be understood by those skilled in theart that various changes or modifications in form and detail may be madewithout departing from the scope and spirit of this invention. Forexample dimensions of the optical sensor unit can be varied (a bigger orsmaller optical sensor unit), thus also the dimensions of themeasurement zone can be bigger or smaller. Also the geometry of theoptical sensor unit can be varied. Even if geometry and/or dimensions ofthe optical sensor unit are changed, preferably the optical sensor unitstands out above the active display area by a distance lower than 0.5cm. Furthermore, applications may be slightly different. For example,the luminance can be measured for each colour, either sequentially or bya combination of sensors with appropriate filters, to measure orstabilise the colour temperature, which is defined by the mixture of theprimary colours, in most cases R, G and B. As another example, themethod and device can be used to stabilise the contrast value of theluminance measured with the described system, and the ambient lightmeasured with a second sensor which does not point at the active area ofthe display, but which points at the room environment or to a non-activeborder of the display. In this case, the display of the image on theactive display area is controlled in accordance with the opticalmeasurement signals in combination with the ambient light measurementsignals.

1. A system for real time correction of light output and/or colour of an image displayed on a display device (1), the system comprising: a display device (1) comprising an active display area (6) for displaying the image, an image forming device (2) and an electronic driving system (4) for driving the image forming device (2), an optical sensor unit (10) comprising an optical aperture (21) and a light sensor (22) having an optical axis, to make optical measurements on a light output from a representative part of the active display area (6) of the image forming device (2) and generating optical measurement signals (11) therefrom, a feedback system (12) receiving the optical measurement signals (11) and on the basis thereof controlling the electronic driving system (4), wherein the optical aperture (21) of the optical sensor unit (10) has an acceptance angle such that at least 50%, alternatively 60%, alternatively 70%, alternatively 75% of the light received by the light sensor (22) comes from light travelling within 15° of the optical axis of the light sensor (22).
 2. A system according to claim 1, wherein the optical measurements are luminance measurements.
 3. A system according to claim 2, wherein light output correction comprises luminance and/or contrast correction.
 4. A system according to claim 1, wherein the optical sensor unit (10) furthermore comprises a light guide (34) between the optical aperture (21) and the light sensor (22).
 5. A system according to claim 4, wherein the light guide (34) is a light pipe.
 6. A system according to claim 5, wherein the light pipe is coated in order to shield ambient light.
 7. A system according to claim 6, wherein the coating (36) does not penetrate into the material of the light pipe.
 8. A system according to claim 4, wherein the light guide is (34) an optical fibre (32).
 9. A system according to claim 1, wherein the optical aperture (21) of the optical sensor unit (10) masks a portion of the active display area (6), while the light sensor (22) does not mask any part of the active display area (6).
 10. A system according to claim 1, wherein the optical sensor unit stands out above the active display area a distance of 5 mm or less.
 11. A system for real time correction of light output and/or colour of an image displayed on a display device (1), the system comprising: a display device (1) comprising an active display area (6) for displaying the image, an image forming device (2), and an electronic driving system (4) for driving the image forming device (2), an optical sensor unit (10) comprising an optical aperture (21) and a light sensor (22) having an optical axis, to make optical measurements on a light output from a representative part of the active display area (6) of the image forming device (2) and generating optical measurement signals (11) therefrom, a feedback system (12) receiving the optical measurement signals (11) and on the basis thereof controlling the electronic driving system (4), wherein the optical aperture (21) of the optical sensor unit (10) has an acceptance angle such that light arriving at an angle with the optical axis of the light sensor (22) which is 10° or greater is attenuated by at least 25%, light arriving at an angle of 20° or greater is attenuated by at least 55% and light arriving at an angle of 35° or greater is attenuated by at least 85%.
 12. A system according to claim 11, wherein the optical measurements are luminance measurements.
 13. A system according to claim 12, wherein light output correction comprises luminance and/or contrast correction.
 14. A system according to claim 11, wherein the optical sensor unit (10) furthermore comprises a light guide (34) between the optical aperture (21) and the light sensor (22).
 15. A system according to claim 14, wherein the light guide (34) is a light pipe.
 16. A system according to claim 15, wherein the light pipe is coated in order to shield ambient light.
 17. A system according to claim 16, wherein the coating (36) does not penetrate into the material of the light pipe.
 18. A system according to claim 14, wherein the light guide is (34) an optical fibre (32).
 19. A system according to claim 11, wherein the optical aperture (21) of the optical sensor unit (10) masks a portion of the active display area (6), while the light sensor (22) does not mask any part of the active display area (6).
 20. A system according to claim 11, wherein the optical sensor unit stands out above the active display area a distance of 5 mm or less.
 21. A system according to any of claims 1 to 20, wherein the representative part of the active display area (6) of the image forming device (2) is less than 1% of the area of the active display area (6) of the image forming device (2), preferably less than 0.1%, still more preferred less than 0.01%.
 22. A method for real time correction of light output and/or colour of an image displayed on a display device (1), comprising: displaying the image on an active display area (6) on the display device (1), making optical measurements on light emitted from a representative part of the active display area (6) and generating optical measurement signals (11) therefrom, controlling the display of the image on the active display area (6) in accordance with the optical measurement signals (11), wherein the step of making optical measurements comprises selecting light such that the ratio between the amount of light used for control which is emitted or reflected from the display area at a subtended acceptance angle of 30° or less to the amount of light used for control which is emitted or reflected from the display area at a subtended acceptance angle of greater than 30° is X:1 where X is 1 or greater.
 23. A method according to claim 22, for carrying out luminance measurements.
 24. A method according to claim 23, wherein light output correction comprises luminance and/or contrast correction.
 25. A method for real time correction of light output and/or colour of an image displayed on a display device (1), comprising: displaying the image on an active display area (6) on the display device (1), making optical measurements on light emitted from a representative part of the active display area (6) and generating optical measurement signals (11) therefrom, controlling the display of the image on the active display area (6) in accordance with the optical measurement signals (11), wherein the step of making optical measurements comprises attenuating light travelling at angles with a normal to the active display area (6) which are equal to or larger than 10° by at least 25%, attenuating light travelling at angles with a normal to the active display area (6) which are equal to or larger than 20° by at least 55%, and attenuating light travelling at angles with a normal to the active display area (6) which are equal to or larger than 35° by at least 85%.
 26. A method according to claim 25, for carrying out luminance measurements.
 27. A method according to claim 26, wherein light output correction comprises luminance and/or contrast correction.
 28. A method according to any of claims 22 to 27, wherein the step of making optical measurements comprises transmitting light from within the active display area (6) to outside the active display area (6). 